A novel two-level simulator for spaceborne Synthetic Aperture Radar (SAR) is presented in this paper, and this simulator consists of two different simulation levels: the raw signal level and the image level and the implementations are discussed in detail. For the raw signal level, a new model with higher computation efficiency based on Graphic Processor Unit (GPU) to generate the radar echoes is proposed. For the image level, an efficient approach to create single-look image is processed. Finally some experimental results by the novel simulator are presented. These results demonstrate not only the validity of the proposed simulator but also the conformability of the two simulation levels.